This invention relates generally to turbomachines and, more particularly, to turbine vanes and blades.
In high performance gas turbine engines, the temperature of the hot gas stream which is generated within a combustor section exceeds the operating temperature capability of any practical material from which the turbine blades and vanes may be fabricated. In order to reduce the temperature of the parts to a point where sufficient strength is maintained, it has become an accepted practice to duct lower temperature, pressurized air from the engine compressor to the turbine components which operate in the hot gas stream environment. One of the most effective methods by which the metal temperatures of such components are cooled is to introduce the cooling air into hollow blades or vanes and then discharge the air into the hot gas stream. This cooling air reduces the component metal temperatures through various heat transfer mechanisms such as convective, impingement, or film-cooling action.
To this end, turbine blades and vanes are generally fabricated in the form of a generally hollow shell with a plurality of cavities and associated dividers forming the inner side thereof. However, the trailing edge portion of the airfoil, because of the constraint of aerodynamic efficiencies, tapers down to a very thin trailing edge. Accordingly, since the cavities cannot extend back to the trailing edge, this solid trailing edge portion will heat up to destructive temperatures unless it is cooled in some way. This cooling is commonly accomplished by the forming of a plurality of trailing edge cooling slots which extend between the internal cavities of the airfoil and the trailing edge thereof for the conduction of cooling air therealong.
Historically, cooling slots have been formed to emerge from the trailing edge portion of the airfoil at substantially the centerline thereof. However, it has been found that higher efficiency is accomplished in the performance thereof by ejecting the trailing edge cooling air on the pressure side of the trailing edge. One of the problems of this design is that with a straight slot formed in the trailing edge, the slot break-out length tends to be too long and the break-out point on the pressure side has a large location tolerance. Since the break-out location is critical to the temperature of the vane trailing edge, a stack-up of tolerances can easily result in a slot which does not satisfactorily cool the vane trailing edge.
One method of reducing the location tolerance is to use a curved air slot. Such a curved slot forms a larger angle with the pressure side of the vane and provides better axial location accuracy as a function of vane thickness and slot location tolerances.
In the case of a vane formed by casting, it is relatively easy to form a curved slot. However, very high turbine temperatures necessitate the use of certain types of material which are not adaptable to being cast but are only available in bar form (wrought form). Accordingly, with such noncast airfoils, the cooling holes and slots must be machined into the part in such a manner as by, for example, electrical discharge machining (EDM). One possible method of machining the curved slot by the EDM method is with the use of a curved electrode. However, with the extremely close tolerance conditions, and with the relatively large depth of the slot, such an exercise would be extremely difficult to accomplish.
It is therefore an object of the present invention to provide a noncast airfoil with an improved trailing edge cooling slot.
Another object of the present invention is to provide a trailing edge cooling slot which emerges on the pressure side of the airfoil trailing edge.
Yet another object of the present invention is the provision in the trailing edge cooling slot of a turbine airfoil for the limiting of the slot break-out length and location tolerance.
Yet another object of the present invention is the provision for accurately and effectively forming a curved trailing edge cooling slot in a noncast turbine airfoil.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.